The present invention relates to high temperature transition joints and more particularly to such a joint adapted for interconnection between a first tubular piece formed from a low alloy steel or carbon steel and a second tubular piece formed from a high temperature alloy.
In many high temperature applications, it is necessary to join together tubular pieces of substantially different characteristics. For example, such applications arise in boiler construction and nuclear power stations where various heat exchanger components, such as steam generators, intermediate heat exchangers, recuperators, boilers, etc., must withstand severe conditions in high temperature environments. Similar problems arise in other industries. For example, petrochemical equipment and chemical processing plants have similar requirements for heat exchangers, steam lines and the like.
In all of these applications, it is usually necessary to form large numbers of interconnections between materials of substantially different characteristics. Usually, one end of the tubular interconnection is formed from a high temperature alloy particularly suited for withstanding high temperature environments encountered in these applications. At the same time, it is necessary for the interconnection or transition joint to withstand the same severe operating conditions of temperature, etc., over extended periods of time.
Under conditions of the type described above, the different types of materials exhibit substantially different characteristics which make it particularly difficult to maintain continuity throughout the transition joint. The existence of different thermal expansion coefficients on opposite sides of any given joint, produce particularly severe stresses which tend to cause cracking or disruption of the joint. Other factors such as deleterious metallurgical changes also exist within such transition joints which further interfere with the maintaining of an effective transition joint or interconnection.
Substantial effort has been expended in the past to develop effective transition joints for such applications. One such approach has been the formation of the joint with continuously changing chemical composition along the length of the joint, one end of the joint being connected to one tubular piece while the opposite end of the joint is joined to a tubular piece of substantially different composition and characteristics. Many different types of material have also been employed to form such transition joints. Heat treatment has also been employed both prior to and following formation of the transition joint in order to better condition the transition joint for withstanding severe operating conditions of the type referred to above.
However, even with such developments and improvements in the area of transition joints, high failure rates have been experienced with transition joints exposed to high temperature conditions. The owners of heat exchanger equipment commonly find it necessary to replace or repair thousands of such transition joints in a single installation. Accordingly, there has been found to remain a need for improved transition joints, particularly for use as an interconnection between low alloy and carbon steel tube or pipe and high temperature alloy tube or pipe.
In connection with the present invention and as described in greater detail below, the term "low alloy steel" is intended to encompass materials such as ASTM Type T-22 composed of 21/4% chromium, 1% molybdenum, 0.15% carbon, balance essentially steel and other similar low alloy ferritic steels. Carbon steel is intended to encompass iron base alloys where the principal alloying addition is carbon in amounts up to approximately 2%. At the same time, the term "high temperature alloy" is intended to encompass both wrought and cast austenitic steels, stainless steels and nickel base alloys for example.